I'll be posting something more significant to this thread soon, but for now I just realized that I already have a small dataset of songs, users, and song ratings sitting around, in the somesongs.org database. If any of you data nerds like fluffy or jast or Jim of Seattle want to hack on some recommendation engines, I'd be happy to export that stuff for you. It looks like I've got ~72k ratings of 3749 different songs by 907 different users. It's not much but it's something. Of course, many of the users aren't around to try out the recommendations and provide feedback, sadly, but there are probably other ways to estimate effectiveness.